Issue #205: detect flings on swipable views

Use velocity tracker to compute the swipe velocity for flings.
There seems to be an issue where swiping from left to right is too
sensitive but not sensitive enough when going right to left.
Issue #205: add OnSwipeDismissListener

Add OnSwipeDismissListener callback that is executed when an item has
been dismissed by swiping. There is no animation to shrink the view, so
it looks abrupt at the moment.
Issue #205: add height shrinking code

Add nasty code to animate layout height to 1. There is a problem in that
our database changes are done asynchronously, so you can see a flash of
the prior item returning to its original state.